over size generator alternator for VFD load???

[Wedoca]

guys, when using generator as back up power. is it critical to oversize the alternator in the generator to run VFDs??? if so why??? I spoke to an VFD tech, and he mention about over size the generator and its got something to do with the high power source impedance of the generator being 6 to 8 times higher than the Utility ... does anyone knows what he mean??

 

[ScottyUK]

Rectifiers like to draw current in short bursts near the peak of the voltage waveform. Generators do not like this. Marginally sized generators especially don't like this. The usual effect is severe flattening of the voltage peak, which is a result of the relatively high internal impedance of the generator source. A good AVR will go some way toward combatting the problem but if you need a clean sinusoid then a bigger machine is generally an option, as is a 12-pulse (or higher) rectifier, or a harmonic trap, or an active filter.


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If we learn from our mistakes I'm getting a great education!

 

[Wedoca]

ScottyUK

this is the waveform taken at the line side of VFD running off of the generator, does this look like what you described ??

 

[ScottyUK]

Yes - if you look at the current waveform there is about 60 degrees of dead time when the current is zero, then about 120 degrees where current flows. I think this drive likely has a DC link reactor which is causing the current to be drawn over a longer period than I expected, but that is possibly also an effect of the highly distorted voltage waveform. The voltage waveform is unusually bad - if you superimpose a sinewave of the appropriate scale onto the trace you will see how much the voltage crest has been flattened by the high current drawn near the voltage crest.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

As the generators get bigger this issue diminishes because the regulators are more capable. What size VFD or motor are we talking about?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Wedoca]

drives are 20Hp and the motors are 10hp

 

[itsmoked]

So you are in the 20~40kW generator realm. You would want to go larger.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Wedoca]

the drive manufacture said that the generator impedance was too large relatively to the drives, thus limiting generators voltage devlier capablity, and since the drive has the voltage supervison settin, therefore the drive try to maintain the voltage, as tradeoff the current went down which explains the frequency drop.

how do I varify what he said is true ??? anyone got an idea???